Section C - Thermal physics and kinetic theory: Key terms and concepts

Convection at work

• Heat transfer through fluids (liquids and gases) via convection occurs when there is bulk motion of the fluid.
• Convection is not possible in solids.
• Convection currents are easier to set up in gases than in liquids.
• Process:
  • Fluid in contact with a hot surface gains heat energy.
  • Heated fluid expands, its density decreases, so it becomes less dense than surrounding fluid and rises.
  • This creates a circulation: the cooler fluid moves in to take the place of the rising hot fluid, forming a convection current.
• Demonstration: aluminium powder in water to visualise convection (Figure 14.3 conceptually):
  1. Place aluminium powder at the bottom of a beaker.
  2. Add water carefully to avoid disturbing the aluminium.
  3. Heat the beaker gently with a flame.
  4. Observe the path of disturbed aluminium particles: they rise through the middle of the water and descend near the cooler sides. The movement of liquid particles drives the rise and fall of the powder.
• Everyday and regional phenomena explained by convection:
  • Breezes: sea breezes and land breezes due to differences in heating between land and sea.
  • Specific heat capacity difference: water has a higher specific heat capacity than soil (land); water ≈ five times the specific heat capacity of soil, so land heats up more than sea during the day.
  • Daytime sea breeze: warmer, less dense air over land rises; cooler air from over the sea moves in to replace it, creating a sea breeze.
  • Nighttime land breeze: the sea remains warmer (water's heat capacity is higher, so water cools more slowly); warmer air over the sea rises and cooler air from land moves toward the sea, creating a land breeze.
• Practical question: Which is better for installing an air conditioning unit, near the roof or near the floor? Reason: cool air sinks, so placing near the floor helps distribute cooler air more effectively.

Conduction at work

• Conduction is the transfer of heat through a solid medium in which particles transfer energy via collisions (and, in metals, via free electrons).
• In metals, free electrons move and rapidly transfer energy along the lattice; in non-metals, energy is transferred mainly by vibrations of atoms (lattice vibrations) since there are no free electrons.
• Compared with convection, conduction is generally a slower process.
• Factors affecting the rate of conduction:
  • The nature of the material (metals vs non-metals; conductivity varies).
  • The thickness of the material (L in the conduction equation).
  • The cross-sectional area (A) available for heat flow.
• Metals are typically good conductors; non-metals are generally poor conductors because non-metals lack free electrons.
• There is a link between electrical and thermal conduction: good electrical conductors are, in general, good thermal conductors.
• The mechanism of thermal conduction is essentially the same as electrical conduction (though carried by lattice vibrations and electrons for heat; by electrons for electricity).
• Practical activity to compare thermal conductivities:
  • Steps:
    1) Obtain rods of different materials but the same length.
    2) Attach a thumb tack to one end of each rod with wax.
    3) Place the other end of the rods into boiling water.
    4) Observe which tack falls off first. The material with higher thermal conductivity will transfer heat faster and cause the tack to detach earlier.
  • Example outcome (from the described setup): aluminium often shows quicker heat transfer (tack falls off sooner) than other metals.
• Additional notes:
  • The experiment illustrates that conduction depends on material properties and cross-section, and that metals usually conduct heat better than non-metals.
  • The wax-tack method provides a simple qualitative comparison of conductivities.

Radiation and radiant energy

• Radiation is the transfer of heat by electromagnetic waves and does not require a medium; it can occur in a vacuum.
• All objects emit radiant energy depending on their temperature; absorption of radiant energy also occurs from the surroundings.
• Detection of radiation: absorbed by skin, thermometers with blackened bulbs, thermopiles, or phototransistors.
• Temperature- and surface-related experiments:
  • Black vs polished surfaces: set up with two metal sheets of the same metal and thickness, one polished (A) and one blackened (B).
  • A radiant source (e.g., Bunsen burner) is equidistant from A and B.
  • The nail attached to the blackened sheet falls off first, indicating that a blackened surface absorbs heat better than a polished surface.
• Leslie’s cube (Figure 14.8): a hollow copper cube with four surfaces (blackened, roughened, white, highly polished).
  • A thermometer/thermopile is placed at a fixed distance from the surface to measure emitted radiation.
  • The greatest radiation is from the black surface; least from the highly polished surface.
• Key implication: good absorbers of radiant energy (black/rough surfaces) are also good emitters of radiant energy.
• The greenhouse effect:
  • Increased use of fossil fuels has raised atmospheric CO₂ concentrations.
  • CO₂ and water vapor do not prevent sunlight from reaching Earth, but they trap infrared radiation emitted by Earth, reducing heat loss to space.
  • This trapping warms the lower atmosphere and surface, contributing to global warming.
  • Potential consequences include climate change and rising sea levels due to polar ice melt.

Practical uses of heat transfer (Section 14.4): solar energy and insulation

• Solar cooking:
  • Solar cookers use a curved reflector with a highly polished surface to collect sunlight and focus it onto a small area.
  • The pot or dish should be placed at the focus to achieve maximum heating.
• Solar water heaters and other solar energy uses:
  • Solar energy can be used for heating water via solar collectors.
• Vacuum thermos/flask:
  • A double-walled glass vessel with a vacuum between the walls minimizes heat loss by convection and conduction (no air between walls).
  • The inner surfaces are silvered to reduce heat loss by radiation.
  • The top and base are made of insulating materials to further reduce heat loss by conduction.
  • The result: the flask keeps hot liquids hot and cold liquids cold (low rate of heat transfer).

Section C — thermal physics and kinetic theory: applications and conceptual questions

• What factors should be considered when selecting outdoor clothing for cold climates? (insulation, warmth-to-weight, wind resistance, breathability).
• What factors should be considered when selecting materials for constructing houses in tropical or hot climates? (insulation, shading, ventilation, heat capacity of materials, glazing, etc.).
• How does radiant energy interact with greenhouse materials (glass/plastic) and with CO₂/H₂O in the atmosphere?
• What is a solar cooker, and how does it work (focusing sunlight onto a pot)? (Figure 14.10 demonstrates hollow plastic top, insulating supports, and slivered/reflective surfaces; double-walled construction parallels in 14.11 for insulation).
• What is a vacuum flask (Figure 14.11) and how does it reduce heat loss?
• Why is water a poor conductor of heat in the context of conduction experiments? (water transfers heat mainly by convection and contains dissolved air; its high specific heat capacity means it stores thermal energy slowly).
• Why are pots typically made of metal but spoons made of wood? (metals are good conductors of heat; wood is a poor conductor, so it is a better insulator and safer to handle hot liquids).
• In conduction experiments, why is a wire gauze placed over a Bunsen burner when heating a beaker of water? (to spread heat evenly, support the beaker, and prevent a direct hot spot that could crack the glass).
• Are other liquids poor conductors of heat? (many liquids are poorer conductors than metals; conduction in liquids relies on molecular interactions and, in many cases, is slower than in solids; water acts as a poor conductor relative to metals, but the exact conductivity varies by liquid).
• Radiative energy travels at the speed of light and does not require contact or mass flow. It can occur in a vacuum. The Sun’s energy travels to Earth through space across a distance of about d o 1.5 imes 10^8 ext{ km}.
• Key idea: the total thermal behavior of a system depends on a combination of conduction, convection, and radiation and how they interact with material properties (conductivity, specific heat, density) and geometry of the system.

Quick reference: core formulas and concepts (highlights)

• Heat flow direction:
  • Heat flows from hotter to colder objects until temperatures equalize: T{ ext{hot}} = T{ ext{cold}}.
• Density concept used in convection:
  • Density is mass divided by volume:
    ho = rac{m}{V}. A region of heated fluid becomes less dense and rises relative to surrounding fluid.
• Conduction: qualitative factors; a quantitative model is often written as
  • q = rac{k A
    abla T}{L} ext{ or } q = k A rac{ riangle T}{L},
• Water and land heat capacity relationship: approximately
  • c{ ext{water}} \, ext{is about} \, 5 \times c{ ext{soil}}.
• Distance Sun to Earth (astronomical):
  • d \, \approx \, 1.5 \times 10^8 \, \text{km}.
• Blackbody radiation relation (conceptual): good absorbers are good emitters (Kirchhoff-like intuition).

These notes cover the major concepts, demonstrations, and real-world applications discussed in the transcript, organized to support exam preparation and quick revision of heat transfer, its modes, and practical considerations in everyday life and engineering contexts.